Field of the Invention
The present invention relates to a method of polishing a substrate, such as a wafer. More particularly, the present invention relates to a method of polishing a peripheral portion of a substrate by pressing a polishing tool, such as a polishing tape, against the peripheral portion.
Description of the Related Art
A polishing apparatus, which has a polishing tool (e.g., a polishing tape or a fixed abrasive), is used for polishing a peripheral portion of a wafer. This type of polishing apparatus is configured to bring the polishing tool into contact with the peripheral portion of the wafer, while rotating the wafer, to thereby polish the peripheral portion. In this specification, the peripheral portion of the wafer is defined as a region including a bevel portion which is the outermost portion of the wafer and a top edge portion and a bottom edge portion located radially inwardly of the bevel portion.
FIG. 52A and FIG. 52B are enlarged cross-sectional views each showing the peripheral portion of the wafer. More specifically, FIG. 52A shows a cross-sectional view of a so-called straight-type wafer, and FIG. 52B shows a cross-sectional view of a so-called round-type wafer. In the wafer W shown in FIG. 52A, the bevel portion is an outermost circumferential surface of the wafer W (indicated by a symbol B) that is constituted by an upper slope (an upper bevel portion) P, a lower slope (a lower bevel portion) Q, and a side portion (an apex) R. In the wafer W shown in FIG. 52B, the bevel portion is a portion (indicated by a symbol B) having a curved cross section and forming an outermost circumferential surface of the wafer W. The top edge portion is a flat portion E1 located radially inwardly of the bevel portion B. The bottom edge portion is a flat portion E2 located opposite the top edge portion and located radially inwardly of the bevel portion B. The top edge portion E1 and the bottom edge portion E2 may be collectively referred to as edge portion. The edge portion may include a region where devices are formed.
In a fabrication process of SOI (Silicon on Insulator) substrate, there is a need to form a vertical surface and a horizontal surface on the edge portion of the wafer W, as shown in FIG. 53. A cross section of such edge portion is achieved by a polishing process as illustrated in FIG. 54. Specifically, while the wafer W is rotated, a pressing pad 300 presses an edge of a polishing tape 301 against the edge portion of the wafer W to thereby polish it. The polishing tape 301 has its lower surface serving as a polishing surface that holds abrasive grains thereon. This polishing surface is disposed parallel to the wafer W. With the edge of the polishing tape 301 located on the edge portion of the wafer W, the pressing pad 300 presses the polishing surface of the polishing tape 301 against the edge portion of the wafer W to thereby form a right-angle cross section as shown in FIG. 53, i.e., the vertical surface and the horizontal surface on the edge portion of the wafer W.
However, as shown in FIG. 55A, the above-mentioned vertical surface may be roughened because the vertical surface is formed by grinding the edge portion of the wafer W with the edge of the polishing tape 301. In addition, as shown in FIG. 55B through FIG. 55D, the profile of the vertical surface may vary before and after replacement of the pressing pad 300 which is a consumable.
In the fabrication process of the SOI substrate, it may be preferable to form, instead of the vertical surface shown in FIG. 53, a reverse tapered surface as shown in FIG. 56. However, it is difficult for the polishing method illustrated in FIG. 54 to form such a reverse tapered surface.
Further, since a contact area of the polishing tape and the horizontal surface differs depending on a position in the horizontal surface of the wafer edge portion, a part of the resultant horizontal surface may be inclined. This problem will be discussed with reference to figures. FIG. 57 is a plan view showing the pressing pad 300 and the polishing tape 301 shown in FIG. 54. The wafer W has a circular shape, while the pressing pad 300 has a rectangular shape. As a result, a contact length (i.e., a polishing area) of the polishing tape 301 and the wafer W is different between a radially-inward region and a radially-outward region in the horizontal surface of the wafer edge portion, as indicated by broken lines shown in FIG. 57. As a result, as shown in FIG. 58, the radially-inward region in the horizontal surface may be inclined.